Medical or other imaging systems may include an array of indirect conversion (scintillator/photosensor) detectors such as a gadolinium oxysulfide (GOS) detectors or direct conversation detectors such as Cadmium Zinc Telluride (CZT) or Cadmium telluride (CdTe) detectors. In a direct conversion detector with a single layer, a common cathode electrode biased at a large negative voltage is located on one side of the detector layer. A pixelated anode biased at ground or close to ground is located on the other side of the layer. Signals from anode pixels are routed through a substrate and/or circuit board to the readout electronics. Cadmium telluride (CdTe) and cadmium zinc telluride (CZT) comprise semiconductor materials with high stopping power, for example, x-ray attenuation, but low mobility and long charge transport time. For example, a single layer direct conversion detector of CZT or CdTe with 1.0 to 5.0 mm thickness typically saturates at about one million to ten million counts per sec per millimeter squared. Silicon (Si) and gallium arsenide (GaAs) comprise semiconductor materials with high mobility and short charge transport time, but low x-ray stopping power, for example, x-ray attenuation.
As an example, a radiation sensitive semiconductor substrate may be partitioned into a plurality of rows of detector elements and a plurality of columns of detector elements to form a two-dimensional array of detector elements. Each detector element is associated with a corresponding electrical contact for transferring the corresponding electrical signal to a readout substrate, which in turn includes electrical contacts for transferring the electrical signal off from the detector. In CZT based detectors, the electrical contacts on the CZT radiation sensitive semiconductor substrate may be gold (Au), platinum (Pt) or Indium (In), depending on the manufacturer of the detector and/or other factors.
The above mentioned energy resolving detectors for X-ray and gamma radiation based on direct conversion materials have been proven as an efficient way to measure photon energies. An incident photon creates a number of electron/hole pairs. Thereafter, electrons and holes typically drift in opposite directions within the electric field supplied by the electrodes. During the drift process, a current is capacitively induced on each electrode attached to the detector system according to the Shockley-Ramo theorem.
Typically, the electrodes are segmented in a pattern of stripes or pixels to provide a spatial resolution of the interaction events which created the electron/hole clouds. As described e.g. in the U.S. Pat. No. 5,677,539, another reason for electrode segmentation is an improved detection of only one sort of charge carriers, as a smaller electrode segment has a relatively low area such that relevant current pulses are only induced if the charges drift within close vicinity to the electrode segment. However, electric field lines can leave the detector crystal at gaps between electrode segments. Charges (e.g. electrons) following the electric field lines can thus be trapped at the surface for a relatively long time, therefore they are no longer contributing to the pulse signal to be measured. One solution of this problem is the implementation of steering electrodes as suggested in the above U.S. Pat. No. 5,677,539, where the steering electrodes are placed within the gap between detector segments and charged such that the electric field lines are always guided towards the collecting (i.e. signal generating) electrodes. However, the implementation of this technology is in some cases not possible (as additional miniaturization or structuring of is required), or the voltage required to drive the steering electrode induces bias currents which negatively affects the noise properties.